Buildings, towers and similarly heavy structures commonly are built on and around a steel frame work. This steel framework generally includes substantially vertical columns supporting substantially horizontal beams, with the beams supporting building floors. A primary structural consideration of such a steel framework is the plural joint connections of the beams to the columns and to one another. An improved structural joint connection is disclosed in U.S. Pat. No. 5,660,017 (hereinafter, “the '017 patent”). However, advanced stress analysis techniques and a study of building collapse mechanisms following seismic events, and also in view of explosive blast events (i.e., terrorist bombings) and subsequent progressive collapse load conditions have resulted in further improvements.
More particularly, during the last decade there have been concerns about how to improve the beam-to-column, and beam-to-beam joint connections of a steel frame building so they will better withstand seismic events (i.e., earthquake), explosions, blasts, massive impacts, and the like as well as other related extraordinary load phenomena. Of particular concern is the prevention of progressive collapse of a building if there are one or more column failures due to the effects of a terrorist bomb blast, vehicular and/or debris impact, earthquake, structural fire, or any other impact and/or heat-induced damaging condition.
Failures in a column, beams, and/or beam-to-column joint connection structures, and beam-to-beam joint connection structures, due to explosions, severe impact and/or sustained fire, have led to progressive collapse of entire buildings. Solutions for this problem have been sought. For example, following the 1994, Northridge, Calif. earthquake, and in addition to the invention set forth in U.S. Pat. No. 5,660,017, a number of other beam-to-column joint connection alternatives to resist failures in beams, columns and/or their associated beam-to-column joint connection structures were suggested or adopted for use in steel frame constructions for improved seismic performance. For example, the reduced beam section (RBS), or “dog bone” joint connection has been proposed, in which the beam flanges are significantly narrowed near the joint connection. This alternative design reduces the plastic moment capacity of the beam allowing inelastic hinge formation in the beam to occur at the reduced section of the beam. This inelastic hinge connection is thought to relieve some of the stress in the welded joint connection between the beam flange and the column flange. An example is seen in U.S. Pat. No. 5,595,040, for Beam-to-Column Connection, which illustrates such “dog bone” connections. But, because the plastic moment capacity of the beam is reduced due to the significant narrowing of the beam flanges, the moment capacity which can be sustained by the beam is also significantly reduced.
Another alternative is illustrated by U.S. Pat. No. 6,237,303, in which slots and/or holes are provided in the web of the beam, in the vicinity of the welded beam-to-column joint connection, in order to provide improved stress and strain distribution in the vicinity of the welded beam flange-to-column flange joint connection. Other post-Northridge joint connections are also identified in FEMA 350-Recommended Seismic Design Criteria for New Steel Moment Frame Building, published by the Federal Emergency Management Agency in 2000. All such post-Northridge joint connections have reportedly demonstrated their ability to achieve significant inelastic rotational capacity to survive a severe earthquake.
However, one important consideration to be noted in contrast to the present invention is that none of these alternative beam-to-column joint connection structures (other than the '017 patent noted above) provide independent beam-to-beam structural continuity across a compromised column. Accordingly, such joint connection structures can provide only a limited amount of post-blast residual gravity load-carrying capacity due to their inability to resist the gravity load demand of simultaneous moment and axial tension, which load interaction is the result of the formation of a “double-span” condition caused by two beams being connected to a common column which is then suddenly or violently removed or otherwise is severely compromised due to explosion, blast, impact or other events. However, in this invention, with the use of gusset plates (or side plates) as taught in the '017 patent, and with proper design, such beam-to-beam continuity across the location of a removed column will be capable of independently carrying gravity loads under such a double-span condition. Additionally, none of these alternatives, except the gusset plates used as taught in the '017 patent, provide any significant torsion capacity or significant resistance to lateral bending to resist direct explosive air blast impingement and severe impact loads. Torsion demands for such joint connection structures are created because while the top flange of a beam is typically rigidly attached to the floor system of a building against relative lateral movement, the bottom flange of that beam is free to twist when subjected to, for example, direct lateral blast impingement loads caused by a terrorist attack. A beam-to-column joint connection structure according to this invention will sustain such double-span conditions as well as demands from severe torsion loads; while also providing advantages in surviving rotational movements, as will be further explained.
That is, a significant consideration is the inelastic durability or life expectancy of a beam, and its beam-to-column joint connection in a steel frame when subjected to cyclic rotational demands of ever increasing magnitude. That is, when a steel frame beam and its beam-to-column joint connection are subjected to cyclic swaying of the building because of earthquake, or perhaps because of impact, the beam experiences cyclic rotational movements relative to the column it connects to, which rotations are typically measured in percentages of a radian. That is, 1 radian equals 360°/2π, or approximately 36.5°.